I got it all mounted and put on the spinner backplate and prop. I then realize that there isn't enough room on the stud for the washer and nut, so I un wind the prop and backplate off the stud(finger tight), but the stud came right out of the engine with the prop and backplate.

Is this normal????
How should I fix this?

Yes it is normal, just screw it back in...

What type of prop are you using? Mine has plenty of room for the nut/washer/spinner backplate--

I'm using an APC 11x7 for break-in.

Do I thread the stud back in and sinch it down tight?

i always did on mine i just double-nuted it and screwed it in. but since then i herd that as the stud tightens down and bottoms out in the crankshaft threads it can cause some run-out. either way is probably fine. as for break-in i would rather use an 11x5 or maybe a 10x7 that way you can still get normal operating rpms just at a richer needle setting. good luck they can be tight!